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For photosynthesis, the chloroplasts of all green plants contain biological solar collectors known as light-harvesting proteins. Because these proteins are not manufactured in the plant cell where they are used, they need to be transported. A specific molecular chaperone ensures they reach their destination. Biochemists at Heidelberg University have now gained elementary knowledge on the structure and function of this chaperone with the help of a variety of methods from structural biology.

The process of photosynthesis takes energy from the sun and converts it into chemical energy, creating oxygen in the process. For this purpose, the chloroplasts of all green plants contain biological solar collectors. These light-harvesting proteins are the most frequently occurring membrane proteins on the planet and are absolutely essential for efficient photosynthesis. Like all membrane proteins, the light-harvesting proteins also have characteristic hydrophobic – i.e. water-repellent – regions with which they are embedded in their target membrane. Until they reach the target membrane, in this case membrane systems in the chloroplasts, a chaperone shields the hydrophobic regions from harmful interactions.

The chloroplast proteins cpSRP43 and cpSRP54 function in this chaperone role for the light-harvesting proteins. “Deciphering the three-dimensional structure of the core complex of these two proteins allows us to draw basic conclusions about how the chaperone functions”, explains Prof. Dr. Irmgard Sinning of the Heidelberg University Biochemistry Center (BZH). The team of scientists working with Prof. Sinning discovered that two protein motifs take part in the interaction between cpSRP43 and cpSRP54, similar to the motifs that play a central role in regulating access to the genetic material in the cell nucleus. While scientists have known for years about the “histone code” involved in the processes in the nucleus, they now face the puzzle of the newly discovered “arginine code” in the chloroplasts.

The Heidelberg scientists conducted their research in close cooperation with colleagues from the Munich Technical University and the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). The researchers combined different structural biology methods in the pursuit of their work. X-ray structure analysis, nuclear magnetic resonance (NMR) spectroscopy, and small angle X-ray scattering were key in revealing the architecture and dynamics of the core complex of cpSRP43 und cpSRP54. In addition, they took advantage of the Biochemistry Center’s protein crystallization platform, which receives support from the Cluster of Excellence CellNetworks at Heidelberg University. The results of the research were published in “Nature Structural & Molecular Biology”.